filtration

Methods Of Separation Of Substances Under Wet Conditions

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Methods Of Separation Of Substances Under Wet Conditions

Some solid particles are insoluble in water. For example chalk powder, dust particles, sand, and tiny pieces of straw are insoluble in water. To separate this type of particles generally we use the following methods

Sedimentation and Decantation

Have you seen pulses being washed in your home? When pulses are kept in a bowl of water, they settle down as they are heavy. However, dirt, insects, tiny pieces of straw, and other lighter impurities float at the top. The water, which contains these impurities, is then poured out and discarded. This process involves two methods: sedimentation and decantation.

The process of separating insoluble solids, suspended in a liquid, by allowing them to settle down is called sedimentation. The solid particles that settle down during sedimentation are called sediments. The process of pouring out the clear upper liquid without disturbing the sediments is called decantation. The liquid above the sediments is called a supernatant.

Methods Of Separation Of Substances Under Wet Conditions 1A mixture of sand and water can also be separated by sedimentation and decantation. The mixture is left undisturbed for some time. Sand, being heavier, settles down and water is poured out into a separate container.

Activity Aim: To observe cleaning of rice by sedimentation and decantation.
Materials needed: A cup of rice and a bowl of water.
Method:

  1. Observe the rice and record your observations. Does the rice look dirty?
  2. Now, mix the rice with water in a bowl and allow it to stand for something.
  3. Without disturbing the layer of rice, decant the water.

Observation: When the mixture is allowed to stand, most impurities came up and floated near the surface, whereas the rice settled down. On decanting the water, we got cleaner rice.
Conclusion: Rice can be cleaned by sedimentation and decantation.

Filtration

The process by which two substances (an insoluble solid and a liquid) are separated by passing the mixture through a filtering device is called filtration.

Filtration is commonly used in our homes. For example, after preparing tea, we filter out the tea leaves using a strainer. Filtration is also done to remove pulp from fresh fruit juice. Water may also contain solid impurities, which can be removed by filtration.

During filtration, the insoluble solid is retained in the filtering device whereas the liquid passes through it. It is important that the particles of the insoluble solid are bigger than the holes in the filtering device for them to be retained in it. A filter paper is a filtering device that has very fine pores in it.

Activity

Aim: To separate a mixture of sand and water by filtration.
Materials needed: Mixture of sand and water, funnel, beaker, flask, and filter paper.
Method:

    1. Fold the piece of filter paper twice to make a cone.
    2. Keep this filter paper cone inside the funnel. Place the funnel into the flask.
    3. Pour the mixture into the flask through the funnel.

Methods Of Separation Of Substances Under Wet Conditions 2

Observation: It is observed that sand is retained in the filter paper but water passes through.
Conclusion: Sand and water can be separated by filtration.

Evaporation

 

The process in which a liquid changes into a gas is called evaporation. In this method, the mixture is heated. The liquid part of the mixture evaporates leaving the solid part behind. For example, a mixture of common salt and water can be separated by evaporation. In fact, evaporating seawater is one of the oldest ways of obtaining salt.

Methods Of Separation Of Substances Under Wet Conditions 3

salt-from-seawater
Fig. Obtaining salt from seawater

Activity

Aim: To separate a mixture of salt and sand.
Materials needed: Mixture of salt and sand, filter paper, burner; test tube, test tube holder and water.
Method:

  1. Take the mixture of salt and sand in a test tube. Add water to it and shake it well.
  2. Salt dissolves in water, whereas sand does not. Sand can be separated using a filter paper.
  3. Now, only salt solution remains in the test tube.
  4. Heat the salt water till all the water changes to vapour, leaving the salt behind.

Observation: Salt is left behind in the test tube.
Conclusion: Two processes, filtration and evaporation, were used to separate a mixture of salt and sand. Thus, sometimes more than one method can be used to separate the components of a mixture.

Condensation

The process in which gas changes into liquid is called condensation. Condensation is the opposite of evaporation. In nature, water vapour in the air condenses to form its liquid form, the dew. Condensation takes place only when water vapour hits a cold surface.

Activity

Aim: To separate both water and salt from saltwater.
Materials needed: Saltwater, kettle, metal plate, a pair of tongs, bowl and burner.
Method:

    1. Take saltwater in the kettle and heat it over the burner.
    2. After some time, steam comes out of the spout of the kettle.
    3. Hold the metal plate (using tongs) above the spout. The steam condenses and changes into water droplets on touching the plate (cold surface).
    4. Collect the falling water drops in the bowl.
    5. Heat until all the water has boiled off.

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Observation: Salt is left behind in the kettle and water is collected in the bowl.
Conclusion: Water and salt could be separated from saltwater by the process of evaporation and condensation.

How can we Separate a Mixture of a Solid and a Liquid using Evaporation

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How can we Separate a Mixture of a Solid and a Liquid using Evaporation

Separation of mixture of a solid and a liquid
All the mixtures containing a solid and a liquid are separated by one of the following processes:

Separation by filtration : The process of removing insoluble solids from a liquid by using a filter paper is known as filtration. Filtration is used separating insoluble substances from a liquid. The liquid passes through the filter paper and collected in the beaker kept below the funnel. The solid particles do not pass through the filter paper and remain behind on the filter paper. The solid substance left behind on the filter paper is called residue. The clear liquid obtained is called filtrate.
Ex.       A mixture of chalk and water is separated by filtration.

Separation by centrifugation : We can separate the suspended particles of a substance in a liquid very rapidly by using the method of centrifugation. Centrifugation is done by using a machine called centrifuge. Centrifugation is a method for separating the suspended particles of a substance from a liquid in which the mixture is rotated at a high speed in a centrifuge.
In the method of centrifugation, the mixture of fine suspended particles in a liquid is taken in a test-tube. The test-tube is placed in a centrifuge machine and rotated rapidly for some time. As the mixture rotates round rapidly, a force acts on the heavier suspended particles in it and brings them down to the bottom of the test-tube. The clear liquid, being lighter, remains on top
Ex.       We can separate the clay particles suspended in water very rapidly by the method of centrifugation. The suspension of clay particles in water is taken in a test tube and rotated very fast in a centrifuge machine. the clay particles settle down at the bottom of the test-tube and clear water remains at the top.

Separation by Evaporation : The changing of liquid into vapours is called evaporation. Evaporation is used to separate
a solid substance that has dissolved in water (or any other liquid). The dissolved substance is left as a solid residue when all the water
(or liquid) has evaporated. The use of process of evaporation for separating a mixture is based on the fact that liquids vapourise easily whereas solids do not vapourise easily. Though evaporation of a liquid can take place even at room temperature but it is very slow at room temperature. Evaporation can be made quicker by heating the solution.
If we have a mixture of common salt and water, then we cannot separate common salt from water by filtration or centrifugation. This is because common salt is completely dissolved in water and not insoluble in it. We can recover common salt from salt-water mixture (or salt solution) by the process of evaporation.
Ex.       The common salt dissolved in water can be separated by the process of evaporation. The solution of common salt and water is taken in a china dish and heated gently by using a burner. The water present in salt solution will form water vapours and escape into atmosphere. When all the water present in the solution of common salt and water gets evaporated, then common salt is left behind in the china dish as a white solid.
How can we Separate a Mixture of a Solid and a Liquid using Evaporation 1 The process of evaporation is used on a large scale to obtain common salt from sea-water.

Purification by crystallisation : The process of cooling a hot, concentrated solution of a substance to obtain crystals is called crystallisation. The process of crystallisation is used for obtaining a pure solid substance from impure sample.

  1. The impure solid substance is dissolved in the minimum amount of water to form a solution.
  2. The solution is filtered to remove insoluble impurities.
  3. The clear solution is heated gently on a water bath till a concentrated solution or saturated solution is obtained (This can be tested by dipping a glass rod in hot solution from time to time. When small crystals form on the glass rod, the solution is saturated). Then stop heating.
  4. Allow the hot, saturated solution to cool slowly.
  5. Crystals of pure solid are formed. Impurities remain dissolved in solution.
  6. Separate the crystals of pure solid by filtration and dry.

Separation by chromatography :
Chromatography is a technique of separating two (or more) dissolved solids which are present in a solution in very small quantities. By using paper chromatography, we can separate two (or more) different substance present in the same solution. This separation is based on the fact that though two (or more) substances are soluble in the same solvent but their solubilities may be different. Some may be more soluble than the others.
Ex.       Black ink is a mixture of several coloured substances which can be separated by paper chromatography.

Separation by distillation : Distillation is the process of heating a liquid to form vapour, and then cooling the vapour to get back liquid. Distillation can be represented as :
Liquid   Vapour (or Gas)
The liquid obtained by condensing the vapour is called ‘distillate’. When the homogeneous mixture of solid and a liquid is heated in a closed distillation flask, the liquid, being volatile, forms vapour. the vapours of liquid are passed through a ‘condenser’ where they get cooled and condense to form pure liquid. This pure liquid is collected in a separate vessel. The solid, being non-volatile, remains behind in the distillation flask.
Ex.       Salt-solution can be separated into salt and water by distillation.